Have a look at Smartthings. (£100 for the hub). This connects to various protocols including Z-Wave and Zigbe and can also interact with other popular home automation hero's such as Nest and Harmony. The Hub just plugs into our wifi router.
Then buy a sensor that is compatible with the Smartthings hub. If it's outside it will need to be weather proof.
The internal light can be either of 2 options. Option 1 (easy) a plug in device, which a lamp then plugs into. Option 2 (a little more messing around) is to put a small back box behind a normal light switch. You'll need to research this as it's not always straight forward as it depends on the wiring of your house.
Next set up a 'rule' in the Smartthings app to switch on the internal light IF it's dark AND there's movement outside. The rules generally switch off the lights after a given period of time, which can be amended within the app.
Frank